Sublattice melting in Cu_2-xSe investigated by single crystal diffuse scattering

Considerable improvements in the efficiency of thermoelectric materials have been achieved by a number of different approaches to reduce their thermal conductivity. This reduction has sometimes been achieved by incorporating complex nanoscale inhomogeneity or loosely bound ions into the crystal structure, which can scatter phonons without affecting their electronic transport, guided by the concept of the phonon-glass electron-crystal. A high thermoelectric figure-of-merit was however recently observed in a very simple crystal structure, the cubic anti-fluorite semiconductor Cu_2-xSe. The very low thermal conductivity in these compounds have been attributed to a sublattice melting of the copper ions, which suppresses the shear modes that would persist even in a phonon glass. This material has thus been dubbed a phonon-liquid electron-crystal. We will present our detailed investigation of this postulated sublattice melting utilizing single-crystal diffuse neutron scattering and the three-dimensional pair-distribution-function analysis as a function of temperature across the structural transition.